1. Field of the Invention
The present invention relates to a method for aligning needle-like structures and an alignment unit.
2. Description of the Related Art
Recent attention-getting carbon nanotubes, which represent needle-like structures, have structures in which sp2 carbons, which constitute a graphite structure, bonded in a plane are rolled into a cylinder on the order of nanometers. The carbon nanotubes have many superior characteristics, and accordingly have been used in various applications. In particular, the carbon nanotubes are often used in electrical materials because of their electrical characteristic in which they can be well conductive or semiconductive. One application of the carbon nanotubes is to use them in MOS transistors.
FIG. 6 is a schematic illustration of a known MOS transistor. For fabrication of the MOS transistor, a plurality of carbon nanotubes 104 are aligned in the same direction on a SiO2 film 11 over a silicon substrate 10, and a source electrode 12, a drain electrode 13, and a gate electrode 14 are formed by photolithography. Then, a high voltage is applied between the source electrode 12 and the drain electrode 13 to break well-conductive carbon nanotubes and allow semiconductive carbon nanotubes to remain. Thus, the MOS transistor is made.
Another application of the carbon nanotubes is to use them as electron sources of field emission displays (FEDs). Carbon nanotubes emit electrons when voltage is applied to them. Many carbon nanotubes are bundled together in the same direction to form an electron emitter, and such electron emitters are two-dimensionally arranged into an FED electron source. Carbon nanotubes have been used in various other applications, and are, in most of the applications, required to be aligned in one direction.
For the alignment of carbon nanotubes, some methods have been disclosed in Japanese Patent Laid-Open Nos. 2000-208026, 2001-93404, 2001-195972, and 2003-197131, and all of which are involved in FED electron sources. In Japanese Patent Laid-Open No. 2000-208026, a material containing carbon nanotubes is encapsulated in a cylinder, and the cylinder is elongated to align the carbon nanotubes in the elongated direction. In Japanese Patent Laid-Open No. 2001-93404 (corresponding U.S. Pat. No. 6,741,017), a conductive paste containing dispersed carbon nanotubes is pressed into many through-holes formed in a ceramic sheet, so that the carbon nanotubes are aligned in a direction perpendicular to the substrate. In Japanese Patent Laid-Open No. 2001-195972, a paste containing dispersed carbon nanotubes is applied to a serrated feature or other physical shapes provided at the surface of a substrate, by screen printing or spin coating. Thus, the carbon nanotubes are aligned in a direction perpendicular to the surface of the substrate. In Japanese Patent Laid-Open No. 2003-197131 (corresponding U.S. Patent Application Publication No. 2003/117065), carbon nanotubes are placed in many small recesses formed in the surface of a metal film to align them in a direction perpendicular to the surface of the metal film.
These methods, however, have disadvantages as follows. The method disclosed in Japanese Patent Laid-open No. 2000-208026 requires complicated production steps for alignment. Furthermore, in order to use the aligned carbon nanotubes for an FED electron source, the method requires additional steps to array the aligned carbon nanotubes in a matrix, thus increasing the number of production steps.
The methods of Japanese Patent Laid-Open Nos. 2001-93404, 2001-195972, and 2003-197131 limit miniaturization of each electron emitter because in the methods, carbon nanotubes are aligned in a direction perpendicular to the substrate so as to be arrayed in a matrix.